Multiply-resonant stub antenna



Jan. 3, 1950 R. s. WEHNER 2,493,514

MULTIPLY-RESONANT STUB ANTENNA Filed Dec. '22, 1945 Y a Sheets-Sheet 1 I N V EN TOR. Poerf 5 W/mer BY Jan. 3, 1950 R. s. WEHNER MULTIPLY-RESONANT STUB ANTENNA 5 Sheets-Sheet 3 Filed Dec. 22, 1945 ATTOF/YE y Patented Jan. 3, 1.950

MULTIPLY-RESONANT STUB ANTENNA Robert S. Wehner, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application December 22, 1945, Serial No. 636,932

Claims.

The present invention relates to antennas adapted to be used either simultaneously or selectively on a plurality of frequencies, and more particularly, to such antennas which are adapted for use on modern aircraft.

An object of the present invention is the reduction of the number of external antennas carried by airplanes.

Another object of the present invention is the provision of a single antenna which may be used simultaneously or successively for transmission or reception of two or more difierent frequencies.

Another object of the present invention is the provision of a compound antenna which may be fed directly from a conventional coaxial transmission line without the need of any intervening matching circuit.

A further object of the present invention is the provision of an antenna having a considerably smaller overall height than previously known antennas.

A further object of the present invention is the provision of a compound antenna which is more satisfactory aerodynamically than a broad-band antenna would be.

The foregoing objects and others which may appear from the following detailed description are attained by providing an antenna having impedance characteristics approximately those of a thick quarter wave stub at, and in the immediate vicinity of, two more or less widely spaced frequencies, the separation of which is arbitrary. The invention includes a thick stub antenna which is electrically a quarter wave in length at the higher of the two frequencies. Into the top of the stub antena is inserted a short circuited piece of transmission line having an electric length equal to one quarter wavelength at the same frequency. The inner conductor of the line section extends beyond the top of the original stub for such a distance that the overall height of the antenna and extension is equivalent to an electrical quarter wavelength at the lower of the two frequencies, when allowance is made for the effect of the line section.

The present invention will be more fully understood by reference to the following detailed description which is accompanied by a drawing in which:

Figure 1 illustrates in elevation and partly in section an embodiment of the present invention;

Figure 2 is a .family of curves illustrating the resistance, and reflective reactance characteris tics of the antenna of Figure 1 at two comparatively widely separated frequencies, while Figure 3 illustrates the same characteristics of a modification of the antenna;

Figure 4 illustrates an application of the present invention to a balanced antenna, and

Figure 5 illustrates a modification of Figure 1 adapted for use onthree frequencies.

' Referring now to Figure 1, reference numeral if! indicates a thick quarter wave stub radiator vertically disposed above a conductive ground plane GP. The stub antenna is arranged to be energized from transmission line TL, having an outer shell I: and an inner conductor l4 directly connected to one end of stub radiator Ill. The outer shell is connected to ground plane GP. Radiator I0 is provided at its upper end with a hollow space l8, Within the hollow space [8 is a concentrically arranged inner conductor 20 connected to the interior of hollow I8 at the lower end. Conductor 20 extends on beyond the end of stub 10 and it is terminated in an extension 22 having a transverse dimension similar to the transverse dimension of stub radiator Ill. The space within hollow l8 and surrounding conductor 20 is filled with a low loss solid dielectric such as polystyrene or polyethylene. Now stub radiator II] has its length H2 so chosen as to be electrically a quarter wave in length at the higher of the two frequencies at which the antenna is designed to be operated. The depth of hollow l 8 is electrically a quarter wave at this high frequency. Thus, only stub it acts as a radiator at the high frequency, the remainder of the antenna being effectively isolated from the source of high frequency by the interposed concentric transmission line section constituted by the hollow l8 and the concentrically arranged inner conductor 20. Since the hollow space is filled with a solid dielectric'having'a higher dielectric constant than air the electrical length is considerably greater than the actual physical length. The extension 22 of conductor 20 has a length H3 such that, taken in conjunction with the length of stub radiator l0 and allowing for the effect of the quarter wave inner conductor extension, the total overall height of the antenna is electrically equal to a quarter wavelength'atthe lower of the'two frequencies. At the low frequency the line section I8, 20 acts merely as an inductance in series with the skin of the stub antenna allowing current to pass through it from the original stub Ill to the surface of the iupper extension 22. At the lower frequency the entire antenna is effective although its physical length is considerably less than a quarter wavelength due to the series inductance interposediby the series insert section which is at the low frequency less than a quarter wave in length. The amount of overall reduction in physical length depends greatly upon the characteristic impedance and physical length of the inserted line section, decreasing in proportion to the characteristic impedance and in proportion to the value of the electrical length of the line at the low frequency.

Figure 2 shows themeasured input impedance of an experimental model of the antenna shownv in Figure 1 and the standing wave ratio in a 36 ohm line connected thereto, while Figure 3 shows the same characteristics for a modification of the antenna. In both antennas the height H2 (Figure l)' of the original stub was kept substantially the same, the characteristic impedance and length of the line section constituted by hollow l8 and inner conductor 20 held constantv at 38 ohms and 9.5 centimeters respectively and the width of the gap between the top of the original stub and the base of the extension member22 held fixed at 1.0 centimeter. pal difference between the models was in the height H: of the extension and, therefore, in the overall height H1.

Hz, the 'height'of the original stub, was chosen with a allowance for the effect of the thickness of the stub in reducing the physical length to give quarter wave resonance in the vicinity of 47.5 megacycles, which frequency was arbitrarily selected as the center of the high frequency pass band.

. The length of line section I8, Was chosen, with a 10% reduction in physical length to allow for end-effect and capacity between the base of The princi-' the extension member 22 and top of stub 10, to

be a quarter wave at the same frequency.

, Two values of the extension Hz were tried, namely, 10.0 and 2.8 centimeters, corresponding to overall heights of 26.1 and 18.6 centimeters respectively for the data in Figures 2 and 8. It

corresponds to Hi/A ratios ranging from .207 to,

.216 although this latter variation is not smooth. From these figures it will be evident that it is possible to secure a stub type antenna having input impedance approximately those of a simple quarter wave stub at and in the immediate vicinity of any twofrequencies no matter how widely these frequencies are separated. If it is desired to use the antenna of the present invention on very high speed airplanes, it will of course be desirable to provide a stream-lined cross section for the radiator and an electrically good base insulator of sufficient strength to support the an tenn-a against the expected wind pressures and vibrational stresses. However, this involves nothing more than ordinary design and the details of such construction have not therefore been shown. .The impedance characteristics of.

the antenna will be modified only slightly by the change in gap capacity and the change of char-' acteristic impedance of the line section due to the change of cross section shape of the antenna and its contain-ed line sections.

The antenna of Figure .4 is designed to be op- 4 erated as a balanced radiator parallel to a ground plane. Here radiators and arranged in an end to end relationship have an overall length equal to a half wavelength at the higher of the two frequencies. Extensions 22 at each end of the radiator extend the electrical length to a half wavelength at the lower of the two frequencies the same wayas they act in Figure 1. Their operation will, it is believed, be fully understood by reference to the description of Figure 1 and will not therefore be repeated. Radiators 30 and 40 are extended, at their adjacent ends in parallel portions 3| and 41 extending into balanced from the unbalanced or asymmetrical .transmission line TL. 1

The further modification in Figure -5 is designed for operation on three separate frequencies between extension 20 and original stub I0 is inserted a third section. Stub In, line section I8,

20 and extension It act as a quarter wave radiator for the secon'df-requency. Line section 18, 20 acts to isolate extension 22 for the second frequency. At the lowest frequency theradiator comprises stub l0, 'ex-tensi0n l0'= and extension 22 with reactances 18, Hand I8, 20' in series therewith.

The compound antenna described above has the following advantages over alternative forms of antennas:

1. Since good input characteristics, approximately those afforded by a simple thick quarter wave stub, can be obtained at any two frequencies regardless of their separation within wide limits, the antennas may be used in applications where no alternative exists since if the separation is very wide a broad-hand antenna large enough to cover the entire range may be impractical both mechanically and 3 aerodynamically while a theoretic-ally possible matching section for matching a simple antenna ata pair of frequencies may require elements which are quite unrealizable in practice. I r

-2. The resonant impedance inthe two pass bands is such that the compound antenna may be directly fed through 'a filter system designed to work into a 36 ohm terminating impedance with- 1 out the need of any matching circuit whatsoever.

3. The resonant impedance in the two pass bands is such that the antenna may be matched to a ohm or other standard coaxial line at the two resonant frequencies by means of "very easily realizable line section matching circuits of the quarter-wave-half-wave type while a simple stub could not be so matched by any practical match ing section at plural frequencies as Widely spaced.

4. Since the overall height of the antenna is considerably less than a quarterwavelength at the lower frequency, a compound stub antenna of the present type is mechanically more practical and has less parasitic drag than the larger of the simple stub antennas it replaces.

5. For a given overall frequency range and a given streamlined form, the compound stub anten'na is more satisfactory aerodynamically than the corresponding broad-band antenna wouldbe since not only is the compound antennags'horter, thus giving. less parasitic 'dra'g but also the compound antenna has much less surface area than a broad-band antenna whereby less lateral area exists to give rise to objectionable lifting forces.

While I have illustrated a particular embodiment of the present invention, it should be clearly understood that it is not limited thereto since many modifications may be made in the several elements employed and in theirarrangement and it is therefore contemplated by the appended claims to cover any such modifications as fall within the spirit and scope of the invention.

What is claimed is:

l. A compound antenna including a radiator extending substantially perpendicular to a conductive ground plane, said radiator having an overall length of a quarter wavelength at the higher of two frequencies at which said antenna is adapted to be operated, said radiator having a hollow portion at its outer end and having an inner conductor therewithin connected at the opposite end of said hollow portion, the depth of said hollow portion being electrically equivalent to onequarter wavelength at the higher of said frequencies and said inner conductor extending from said hollow portion a distance such that the electrical overall length of said antenna is effectively substantially one quarter wavelength at the lower of said operating frequencies.

2. A compound antenna including a radiator extending substantially perpendicular to a conductive ground plane, said radiator having an overall length of a quarter wavelength at the higher of two frequencies at which said antenna is adapted to be operated, said radiator having a hollow portion at its outer end and having an inner, conductor therewith connected at the opposite end of said hollow portion, the depth of said hollow portion being electrically equivalent to one quarter wavelength at the higher of said frequencies and said inner conductor extending from said hollow portion a distance such that the electrical overall length of said antenna is substantially one quarter wavelength at the lower of said operating frequencies, the extending portion of said inner conductor having a diameter substantially equal to that of said radiator.

3. A compound antenna for operation at a pair of widely separated frequencies including a large diameter radiator having energizing means coupled to one end of said radiator, said radiator having an overall length of a quarter wavelength at the higher of said two frequencies, said radiator having a hollow portion at its other end and having an inner conductor therewithin connected at the inner end of said hollow portion, the depth of said hollow portion being electrically equivalent to one quarter wavelength at the higher of said frequencies and said inner conductor extending from said hollow portion a distance such that the electrical overall length of said antenna is substantially a quarter wavelength at the lower of said frequencies.

4. A compound antenna adapted for operation at two widely spaced frequencies including a large diameter radiator extending normal to a conductive ground plane and having an overall length equal to a quarter wavelength at the higher of said two frequencies, a coaxial transmission line having an inner conductor connected to the lower end of said radiator and the outer shell thereof connected to said ground plane, said radiator having a hollow at its outer end and having an inner conductor therewithin connected at the bottom of said hollow portion, said hollow portion being filled with a solid dielectric, the depth of said hollow portion being electrically equivalent to one quarter wavelength at the higher of said frequencies, said second inner conductor extending from said hollow portion a distance such that the electrical overall length of said antenna is substantially one quarter wavelength at the lower of said frequencies.

5." A compound antenna adapted for operation at two widely spaced frequencies including a large diameter radiator extendingnormal to a conductive ground plane and having an overall length equal to a quarter wavelength at the higher of said two frequencies, a coaxial transmission line having an inner conductor connected to the lower end of said radiator and the outer shell thereof connected to said ground plane, said radiator having a hollow at its outer end and having an inner conductor therewithin connected at the bottom of said hollow portion, said hollow portion being filled with a solid dielectric, the depth of said hollow portion being electrically equivalent to one quarter wavelength at the higher of said frequencies, said second inner conductor extending from said hollow portion a distance such that the electrical overall length of said antenna is substantially one quarter wave-' length at the lower of said frequencies, the extending portion of said inner conductor having a diameter equal to that of said radiator.

6. A compound antenna for operation at a number of widely separated frequencies including'a pair of radiators in an end to end relationship and having energizing means coupled to the adjacent ends of said radiators, the overall length of said radiators being a half wavelength at the higher of the operating frequencies, each of said radiators having a hollow portion at their remote ends and having an inner conductor therewithin connected at the inner end of said hollow portion, the depth of said hollow portions being electric'ally equivalent to one quarter wavelength at the higher of said frequencies, and said inner conductors extending from said hollow portions a distance such that the electrical overall length of the antenna is substantially a half wavelength at the lower of said frequencies.

7. A compound antenna for operation at a number of widely separated frequencies including a pair of radiators in an end to end relationship and having energizing means coupled to the adj acent ends of said radiators, the overall length of said radiators being a half wavelength at the higher of the operating frequencies, each of said radiators having a hollow at their remote ends and having an inner conductor therewithin connected at the inner end of said hollow portion, the depth of said hollow portions being electrically equivalent to one quarter wavelength at the higher of said frequencies, and said inner conductors extending from said hollow portions a distance such that the electrical overall length of the antenna is substantially a half wavelength at the lower of said frequencies, said radiators extending as a pair of parallel conductors at their adjacent ends into a conductive sleeve having a depth equal to a quarter wavelength, said radiators being electrically connected together and to said sleeve at the remote end of said sleeve, a transmission line having a conductor passing through one of said parallel conductors and across to the other of said conductors at the adjacent ends of said radiators.

8. A compound antenna for operation at a number of widely separated frequencies including a pair of radiators in an end to end relationship enema-14 7 and having en'ergiaingineans coupled to the-ad-f jacentaendsof-said radiators, the overall length of said radiators being a half wavelength at the higher of the operating frequenciea each of said radiators having a hollow portion at their remote ends and having an inner conductor "therewithin connected at the inner end of said hollow "porition the depth of said hollow portions'being electrically equivalent .to one quarter wavelength at the higher of said frequencies, and said inner conductors extending from said hollow portions a-distance such that the electrical overall length of the antenna is substantially a half wavelength at the lower of said frequencies, said radiators extending asa pair of parallel-conductors at their adjacent ends into a conductive sleevehaving a depth equal to a quarterwavelengtnisaid radiators being electrically connected together and to said sleeve at'the remote end ofsaid sleeve, "a'

transmission line having a conductor passing through one of said parallel conductors and across to the other of said conductors at the adjacent ends of said radiators, another conductor of said transmission line being connected vto .said sleeve. 7 '9. A compound antenna for operation at a number of widely separated frequencies including a large diameter radiator having energizing means coupled to one end of said radiator, said radiator having an overall length of a quarter Wavelength at the higherof said two frequencies,

said radiator having a hollow at its other end and having an inner conductor therewithin connected to the inner end of said hollow portion, thefdepth of said hollow portion being electrical- 1y equivalent to one quarter wavelength at'the,

higher of said frequencies and said inner conductor extending from said hollow portion a distance such that the electrical length of said radiator and extension is substantially a quarter wavelength at another of said frequencies, saideitens'ion' also having ahollow at its fremote'end and having another inner' con-ductorconnected at the inner end of said hollow portion-e f said extension, thedepth of said se cond ho'llow portion being electrically equivalent to a quarter wavelength at said other ofsa'id frequencies and said ether 'inner conduc'torextendi-ng a distance such that the-electrical overall length of'sa id antenna is substantially a quarter wavelength'at the lower of said frequencies.

10. An antenna including a conductor extending vertically from a conductive 'shee't, said conductor having an-overallleng-th er a quarter wavelength at a given frequency-at which the antenna is adaptedto'be operated, said conductor having a'hollow portion at the end remote from said conductive sheet, a conductive element arranged insaid hollow portion, said conductive element being connected to said conductor at the "innermost end of said hollow portion, said hollow port i'onihaving an electricallengthof a quarter wavelength at said given frequency, and said conductive element extending beyond said hollow-portion a-distance at which the overall=electrical length of said-antenna is substantially:onequarter wavelength of a frequency lower'than said given frequency and at'which the'antenna is also adapted to be operated.

ROBERT S.

REFERENCES :CITED The following references are of record in the fileof this patent:

UNITED STATES PATENTS Bruce Mar. '9, 1943 

